WASHINGTON (AP) -- President Barack Obama and Defense Secretary Leon Panetta both called Afghan President Hamid Karzai to express their condolences after an American soldier in Afghanistan wandered off base and allegedly gunned down 16 villagers. Panetta vowed to "bring those responsible to justice."
In a statement released Sunday by the White House, Obama said, "This incident is tragic and shocking, and does not represent the exceptional character of our military and the respect that the United States has for the people of Afghanistan."
Panetta said a full investigation was already under way.
A U.S. official said the suspect is a conventional soldier from Joint Base Lewis-McChord, Wash. He was assigned to support a special operations unit of either Green Berets or Navy SEALs engaged in a village stability operation.
Such operations are among NATO's best hopes for transitioning out of Afghanistan. They pair special operations troops with local villagers chosen by village elders to become essentially a sanctioned, armed neighborhood watch.
The official spoke on condition of anonymity because the investigation was still ongoing.
Afghans have expressed doubt that a single soldier could have carried out the shootings. And on Monday the militant Taliban vowed to avenge the shootings, saying on a website that "American savages" were responsible.
Gen. John Allen, the top U.S. commander in Afghanistan, issued a statement pledging a "rapid and thorough investigation" into the shooting spree, and said the soldier will remain in U.S. custody.
U.S. officials said the service member was being detained in Kandahar. One U.S. official said the soldier, an Army staff sergeant, was believed to have acted alone and said initial reports indicated he returned to the base after the shooting and turned himself in.
The shootings come at a particularly sensitive and critical time for the U.S., just as violence over the burning of Muslim holy books at a U.S. base was starting to calm down. Sunday's incident could further fuel calls for a more rapid withdrawal of U.S. troops from Afghanistan.
The destruction of Qurans in a fire pit used to burn garbage last month sparked violent protests that killed some 30 people. Six U.S. service members were killed in attacks by Afghan security forces since that incident, which U.S. officials have apologized for and said was accidental.
AP Intelligence Writer Kimberly Dozier contributed to this report.